As is well known, aluminum alloys form a tenaciously adherent aluminum oxide (Al.sub.2 O.sub.3) film when exposed to atmospheric air at ambient temperatures. The formation of the aluminum oxide film essentially prevents further oxidation of the aluminum alloy and thus serves as a corrosion inhibitor. When better corrosion resistance is required, aluminum alloys are often anodized, which is a chemical conversion at the surface of the aluminum alloy to form a thick and uniform aluminum oxide film. Because the anodizing process involves an applied voltage while the alloy component is in an electrolytic bath, the uniformity of the aluminum oxide film is significantly diminished if the component has a complex geometry. Though a great improvement in corrosion resistance is not necessarily attained, the resulting oxide film provides an adhesive base for organic coatings and paints.
Chromate conversion coating is another form of chemical conversion which has been traditionally employed as a corrosion protection mechanism for aluminum alloys, particularly when the alloy will be used in a highly corrosive environment, such as a heat exchanger for an automobile. The process involves exposing the aluminum alloy to chromic acid, together with an activator such as ferri-cyanide or molybdate, so as to form a chromium oxide barrier layer containing chromate ions, which serve as an inhibitor of corrosion for aluminum alloys. The barrier layer not only improves the corrosion resistance of the article on which it is formed, but also provides an adhesive base for subsequent coating processes.
However, due to the toxicity of chromium, environmental laws require that the presence of chromium in the waste stream discharged from a manufacturing facility be substantially reduced. Environmental and health concerns, coupled with the high costs associated with limiting the amount of chromium released to the environment from a chromium conversion process, have prompted the search for a suitable non-chromium, corrosion-resistant coating for aluminum alloys.
One such coating is discussed in Australian Patent No. We 88/06639, in which an aluminum alloy is exposed to an acidic mixture of hydrogen peroxide (H.sub.2 O.sub.2) and cerous chloride (CeCl.sub.3.xH.sub.2 O; also known as cerium chloride) to form a cerium oxide (CeO.sub.2) coating on the aluminum alloy. The cerium oxide coating serves as a barrier layer whose corrosion resistance is superior to that of aluminum oxide, so as to enhance the corrosion protection of the underlying aluminum alloy. The Australian patent suggests that the coating of cerium oxide, also referred to as cerium dioxide, ceric oxide and ceria, is generated without first forming a uniform, significant aluminum oxide film on the alloy. Though the use of a cerium oxide coating provides the benefits of a corrosion-resistant coating without the toxicity of chromium, a disadvantage with the process disclosed by the Australian patent is the use of hydrogen peroxide, which is considered hazardous in a manufacturing environment because it poses a dangerous fire and explosion risk, particularly at high concentrations.
Thus, it would be desirable to provide a method for forming a suitable barrier layer on aluminum alloys for the purpose of enhancing the corrosion resistance of aluminum alloys, while avoiding the use of toxic, polluting and hazardous compounds, such as chromium and hydrogen peroxide.